1. Field of the Invention
The present invention relates to a method for press molding a thermoplastic resin. More particularly, the present invention relates to a method for efficiently press molding a thermoplastic resin to obtain a molded article having good appearance and less deformation or warp in a shorter molding cycle.
2. Description of the Related Art
To produce a molded article of a thermoplastic resin, there are known press molding comprising pressing and cooling a plasticized resin melt in a mold and injection molding comprising supplying a resin melt in a closed mold.
However, each of the conventional molding methods has the following drawbacks.
First, in the former press molding method, a nozzle for supplying the plasticized thermoplastic resin should be inserted in and removed from the mold in each molding cycle, so that the molding cycle becomes longer. In the press molding, since it takes a long time from start of the resin melt supply to start of the molding by a pressing pressure, uneven gloss or small wrinkles tend to be formed on a surface of the molded article.
In the latter injection molding method, since the resin melt is supplied under pressure into a cavity of the closed mold, orientation of the resin is generated and residual strain remains in the molded article, so that the molded article tends to suffer from deformation such as twisting or distortion. In addition, since large residual strain remains at a part of the molded article near a gate of the mold, such part is physically weak.